We have a bad sewer gas problem emitting primarily from one A/C vent in a hall bathroom. Of course if the fan is off on the A/C it will eventually fill all the ducts and come out in every room. The house is built on a slab, the A/C ducts are in the floor and the returns are in the ceiling. Over the past month or so I have repaired the drain in the shower in the master bathroom, replaced the wax ring on the toilet and the sanitary T for the sink in the hall bathroom. All the vents are open to the roof, and I removed the A/C condensate line from the drain in the garage to make sure it was not pulling gas up from there. When we do laundry, the drain in the garage overflows sending water running out of the A/C/hotwater heater enclosure. The house was built in 1983 and we have lived here for 8 years without a plumbing problem. When we moved here, all the A/C outlets in the floor had rusted out and we had to replace them. Heavy rains would cause them to fill with water, so we replaced the galvanized boxes with aluminum. Am I looking at extensive repairs, or am I over looking something simple? Plumbers want to do back pressure tests etc. Very expensive! This started after a visit from the family, and has gotten worse since I made the repairs.

First, I think this question should be posted in HVAC Forum, not plumbing.

As far as I know (... and I am a plumber ), it is not good idea to run your ducts under slab. That's the reason for your registers to succumb to rust. If they rusted out so did your ducts. It is very possible the ducts are flooded with ground water. Water stands on the bottom of the ducts and when you activate fan, it sends foul odor through your system.

It would be a good idea to do video inspection of your ducts to determine their condition.

Question: I do not understand where is the water coming from (from your AC closet) . Can you post a photo of the closet ? Thanks

Question: I do not understand where is the water coming from (from your AC closet) . Can you post a photo of the closet ? Thanks[/QUOTE]

The main reasoning for posting on plumbing is because the odor started in the AC after the plumbing repairs were complete and gave me cause to wonder if I had just moved the problem farther down the line. The odor is not just a foul smell, it is sewer gas. At this time the AC is running, and there is a faint odor coming from the AC vents in both bathrooms, but the kitchen, LR, all three BRs, DR, entryway vents emit fresh smelling cold air. The AC closet and the utility room share a common wall and the floor drain in the closet accommodates the condensate line from the AC and the pressure overflow from the hot water heater. I guess the standpipe from the laundry also connects to the line that serves the floor drain. My wife has complained over the last couple of years that the machine does not rinse properly and her laundry does not smell as fresh as it did. New machines and other solutions have not satisfied her. I would think that once the wash water is out of the way everything would be OK. Looks like I keep adding to my problems, but the main issue is the sewer gas in the AC, and the water from the floor drain in the AC closet. Thanks for all your help.